NK,Gr.A Shipbuild Steel Plates

 

 

 

Microstructure Standards after Heat Treatment

 

Quenching Microstructure Standards: After quenching, the normal metallographic microstructure of medium - carbon steel should be martensite and retained austenite. Martensite is in the form of needles or laths and has a darker color. The amount of retained austenite is relatively small and is distributed among the martensite. For example, when the quenching temperature and cooling rate are appropriate, medium - carbon steel can obtain a fine and uniform martensite microstructure, which enables the steel to have high hardness and strength.

 

Tempering Microstructure Standards: The metallographic microstructure of medium - carbon steel after tempering will change according to the tempering temperature. After low - temperature tempering (150 - 250°C), the microstructure is mainly tempered martensite, and its needle - like or lath - like form still exists, but the hardness is slightly reduced and the toughness is improved. After medium - temperature tempering (350 - 500°C), the microstructure transforms into tempered troostite, which is extremely fine cementite particles distributed on a ferrite matrix. At this time, the steel has better comprehensive mechanical properties. After high - temperature tempering (500 - 650°C), the microstructure is tempered sorbite, and the cementite particles are coarser and evenly distributed on the ferrite matrix. The toughness of the steel is further improved, and the strength and hardness are relatively reduced. These metallographic microstructure standards under different tempering conditions can help determine whether the heat treatment of medium - carbon steel is qualified.

 

 

 

 

 

Grain Size Standards

 

Grain size is one of the important indicators for evaluating the metallographic microstructure of medium - carbon steel. Under normal circumstances, the grain size of medium - carbon steel should meet the corresponding standard requirements. Generally, the comparison method or the intercept method is used to evaluate the grain size. The comparison method is to compare the grain size of the sample with the standard grain size atlas to determine the grain size level. The intercept method is to measure the average intercept of grains to determine the grain size. For medium - carbon steel, it is generally hoped that the grain size is fine and uniform. For example, in some high - quality medium - carbon steel products, the grain size is required to reach level 8 - 10 (the larger the grain size level number, the finer the grains), which can ensure that the steel has good comprehensive properties. In metallographic inspection, if the grain size is found to be abnormally coarse or non - uniform, it may be caused by improper heat treatment such as overheating or burning, or problems with the quality of raw materials.

 

 

 

 

 

 

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